On account of their excellence in a variety of characteristics such as impact resistance and fabricability, rubber-modified styrenic resins (HIPS) are used extensively in electrical, electronic, office and communication equipment. In recent years, there is a tendency to omit coating of the surface of molded articles from the viewpoint of more efficient recycling of molding materials and this has created a strong demand for improvement of surface characteristics such as appearance and scratch resistance.
The impact resistance of HIPS is obtained by dispersing particles of rubbery polymers in styrenic resins. The rubber particles, however, are responsible not only for the uneven surface of molded articles but also for lowered gloss and scratch resistance and thus disqualify HIPS for application to end uses requiring good surface appearance and scratch resistance. A number of methods, for example, size reduction of rubber particles and control of particle size distribution, have been proposed to improve the aforementioned surface characteristics of articles molded from HIPS. These methods, however, do not produce a good balance of impact resistance, rigidity and surface appearance and, in addition, present a critical problem of fundamental difficulty in imparting scratch resistance to the surface of molded articles.
On the other hand, addition of lubricants such as polysiloxanes for improving the slipperiness of the surface has been proposed as a means for improving the scratch resistance of the surface of molded articles. This method is not always sufficiently effective for improving the scratch resistance and the additives thereby incorporated tend to produce poor surface appearance and stain molds. Compounding scratch-resistant materials such as methacrylic resins with ABS resins is also known as a means to improve simultaneously impact resistance, surface appearance and scratch resistance. The resulting formulations, however, suffer marked deterioration of fabricability.
Japan Kokai Tokkyo Koho No. Hei 6-25507 (1994) discloses rubber-modified styrenic resin compositions (rubber-reinforced methacrylate-styrene copolymer resin compositions) containing rubbery elastomers as dispersed particles and copolymers of styrenic monomers and (meth) acrylate monomers as continuous phase and showing excellent appearance and other surface characteristics. This method, however, necessitates copolymerization of a large quantity of (meth)acrylate monomers for sufficient improvement of the surface characteristics, in particular, scratch resistance and eventually leads to deterioration of fabricability. In order to maintain the fabricability, it becomes necessary to control the glass transition temperature of the matrix phase and, in turn, it becomes necessary to additionally copolymerize butyl acrylate. The overall result is lowered heat resistance, narrower latitude in practical use of molded articles and higher cost.
In an attempt to solve these problems, Japan Kokai Tokkyo Koho No. Hei 6-157863 (1994) discloses a method which comprises mixing the aforementioned rubber-reinforced methacrylate-styrene copolymer resins and HIPS at a specified ratio and utilizing the mixture. This method, however, is not very effective for improving the scratch resistance and manifesting high rigidity because the rubbery elastomers are dispersed as particles in the methacrylate-styrene copolymer resins.
As described above, there is a demand for development of rubber-modified styrenic resin compositions with good fabricability and surface characteristics such as appearance and scratch resistance and additionally with a good balance of impact resistance and rigidity.
The present inventors have conducted extensive studies to meet such a demand, found that the aforementioned problems can be solved by mixing specified rubber-modified styrenic resins and rubber-free styrene-(meth) acrylate ester copolymer resins at a specified ratio and completed this invention.
Accordingly, an object of this invention is to provide rubber-modified styrenic resin compositions of good fabricability with excellent surface characteristics such as appearance and scratch resistance and high impact resistance and rigidity.